Increasing evidence suggests that mitochondrial defect is involved in neurodeorenerative diseases. However, the question whether the mitochondrial defects are consequences or a cause of neurodegeneration has not been answered. We propose to answer this question in a transgenic mouse model for amyotrophic lateral sclerosis (ALS). ALS is a fatal neurodegenerative disease that causes motoneuron degeneration, paralysis and death. Mutations in Cu, Zn superoxide dismutase (SOD1) gene are one cause of this disease. Using transgenic mice that express mutant SOD1 and develop this disease, we have begun to investigate the relationship between the disease progression and mitochondrial changes. We have found a massive mitochondrial degeneration at early disease stages, by morphological analysis. To determine how mitochondrial function is affected, we propose to measure mitochondrial enzyme activities at different disease stages using blue native polyacrylamide gel electrophoresis (BN-PAGE) combined with histochemical assays. If an early decline in mitochondrial enzyme activities in disease progression is observed, it will strengthen the view that a rnitochondrial defect plays a role in motoneuron degeneration. If the mitochondrial defect is observed late in the disease progression, it will suggest that mitochondnial degeneration is a consequence of neuronal degeneration. To differentiate whether mitochondria in motoneurons are selectively affected, we will use histochemical reactions in tissue sections. To explore the mechanism by which mutant SOD1 damages mitochondria, we will determine whether mutant SOD1 is associated with mitochondria using biochemical and morphological approaches. A direct association of mutant SOD1 with mitochondria will suggest that mitochondria are damaged directly by the toxicity of mutant enzyme.